Atmospheric Pressure, Wind, Circulation

Summer (wet) monsoon

*During the summer, the direction of surface winds reverses. With the sun directly overhead, average temperatures over the Asian landmass increase significantly. *As a result, the Intertropical Convergence Zone shifts northward over Asia bringing low pressure and a subtropical high is now over the Indian Ocean. *The warm winds from this high pressure anticyclone pick up moisture as they pass over the Indian Ocean. The air is pushed upward by the warmth of India and then pushed up further by the Himalayas. *As the air rises, it cools and precipitation occurs. The high precipitation of the monsoonal rainy season lasts from June to September.

La Nina conditions

*La Niña occurs when abnormally high pressure occurs over the eastern tropical Pacific, which intensifies trade winds. *As a result, the cool deep upwelling waters off the coast of South America tend to dominate the tropical Pacific and migrate further west than in normal years. The sea surface temperature in the eastern tropical Pacific averages between 1-4° Celsius (2-7° Fahrenheit) lower than normal during a La Niña event. Remember that this cool water upwelled from a great depth is nutrient-rich. *During La Niña years precipitation is abundant in the western tropical Pacific, but scarce in other parts of the world.

Seasonal migration of pressure systems

*Spring and fall equinoxes: the sun is directly overhead at the Equator. Given that the most intense radiation is located at the Equator, that is where the Intertropical Convergence Zone is located. *Winter solstice (Northern hemisphere): the sun is directly overhead at the Tropic of Capricorn. Because this zone receives the most intense radiation, this is where the Intertropical Convergence Zone is located. All systems to the north migrate in a southerly direction. *Summer solstice (Northern hemisphere): the sun is directly overhead at the Tropic of Cancer. Intertropical convergence zone located at or near the tropic of cancer during the northern hemisphere summer.

Global circulation

*The unequal heating of the tropics and the poles. *Global circulation works to balance out the unequal heating that occurs between the tropics and the poles. *Moving from the tropics to the poles, zones of low pressure generally alternate with zones of high pressure, so, low then high then low, and so on. *Air that rises in the equatorial trough eventually cools and later sinks to the north and south creating subtropical high.

Pressure gradient force

-The difference in pressure between two such areas that results in air flow is referred to as the pressure gradient force (PGF). -The greater the pressure gradient (the difference between the highs and lows), the stronger the force and thus the more intense the resulting winds. -The process of airflow is significantly affected by the pressure gradient because the greater the pressure difference between two places, the faster the air flows from high to low to equalize the pressure.

Factors affecting large-scales winds

1) Unequal heating 2) Coriolis force 3) Frictional force

Key concepts: Variables that influence large-scale winds

1)Air generally flows from high pressure to low pressure. 2)Large‐scale atmospheric circulation is caused by the unequal heating of the tropics and poles. The process of airflow begins at the Equator because of convection. 3)Isobars are isolines that connect points of equal atmospheric pressure. 4)The speed of airflow is determined by the pressure gradient force. The steeper the gradient, the stronger the winds. 5)The Coriolis force causes air moving toward the Equator to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 6)Features on the Earth's surface, such as mountains, forests, and buildings, create a frictional force that acts opposite to the wind's direction. 7)The combined effect of the pressure gradient force, Coriolis force, and frictional force causes a spiral motion of air in both low‐ and high‐pressure systems.

Key concepts: atmospheric pressure systems

1)Air pressure refers to the weight of air distributed on the surface of Earth. It generally decreases with increasing altitude. 2)Low‐pressure systems are called cyclones and consist of rotating air masses that lift air from the surface. In the Northern Hemisphere, these systems rotate counterclockwise, whereas they rotate clockwise in the Southern Hemisphere. 3)Low‐pressure systems are usually associated with clouds and precipitation. 4)High‐pressure systems are called anticyclones and consist of rotating air masses that descend toward the surface. In the Northern Hemisphere, these systems rotate clockwise, whereas they rotate counterclockwise in the Southern Hemisphere. 5)High‐pressure systems are usually associated with clear skies. 6)On weather maps, isobars indicate areas of equal air pressure. 7)Winds flow from high to low pressure.

High pressure system (anticyclones)

A rotating column of air that descends toward the surface of earth where it diverges. *Anticyclones: The vertical flow of air consists of cooler, descending air at the center of a high that diverges (spreads apart) at the surface. -Because air sinks in the center of a high, the highest pressure is in the central part of the system. -The horizontal air flow around the center of a high is clockwise in the Northern Hemisphere. -High pressure centers are generally associated with fair weather.

Low pressure system (cyclone)

A rotating column of air where air converges at the surface and subsequently lifts. *Cyclone: The vertical flow of air consists of warmer, ascending air that converges at the center of a cyclone. -Because air rises in the center of a low, the central part of the system has the lowest pressure. -The horizontal flow around the center of a low in the Northern Hemisphere is counterclockwise (looking at it from above). -Low pressure centers are generally associated with cloudy or stormy weather.

Oceanic circulation

A second way that heat is transferred around the globe. Ocean circulation more or less mirrors atmospheric circulation, except where the continents block the water's movement. In fact, surface ocean currents are driven largely by winds. In the Northern Hemisphere, ocean currents move in a general clockwise direction. In the Southern Hemisphere, they move in the opposite direction or counterclockwise.

Equatorial low-pressure trough

Air at the equator is warmed due to the year-round receipt of direct sunlight. As air is warmed, molecules spread out - the warmed air is less dense. This creates a zone of low pressure and air near the equator, converges at the surface and rises.

Frictional forces

Air flow is basically unimpeded in the upper atmosphere, this is not the case nearer the earth's surface. Wind is slowed by frictional forces. These frictional forces are created by the drag that results from features, like mountains, forests, and buildings that serve to slow down wind. *Frictional forces act to reduce the speed and alter the direction of winds. As a result, winds will cross isobars below 1,500 meters. With increasing distance from the surface, the influence of frictional forces is lessened. In the upper atmosphere, wind will flow parallel to isobars and become circular as it flows from high to low pressure

Factors that influence air pressure

Altitude; temperature *Air pressure at any altitude is equal to the weight of a column of air extending above that altitude to the top of the atmosphere. *Average atmospheric pressure decreases with increasing altitude above the earth's surface.

El Nino (Pacific Ocean)

An El Niño occurs when the normal (equatorial) circulatory system in the Pacific is reversed. This reversal results in drought in the western Pacific and strong storms in the eastern Pacific. This weather reversal occurs because the atmospheric pressure in the eastern Pacific is relatively low, whereas it is relatively high in the western Pacific. Warm surface waters build up in the eastern tropical Pacific. A pool of warmer water also shifts from the western side of the tropical Pacific (off the coast of Australia and the Indonesian Archipelago) to the east (just off the coast of Ecuador and Peru [use a world Atlas to locate]).

Hadley cell

At low latitudes (from about 0° to 30° north and south latitude), the flow to and from the Intertropical Convergence Zone and subtropical high forms a three-dimensional atmospheric circulation cell, known as a Hadley cell. A Hadley cell consists of rising air at the Equator, which then flows to the north in the Northern Hemisphere and sinks in the subtropical high. This air convection cell is mirrored in the Southern Hemisphere, where the rising air flows to the south before descending the subtropical high.

Subtropical high pressure system

Band of high air pressure, calm winds, and clear skies that exists at about 25° to 30° N and S latitude. As air in the equatorial trough rises from the Earth's surface, it spirals upward to the upper part of the troposphere. During this process, the temperature of the air drops as it rises. Descending air within the STH is dry because much of its moisture was lost as precipitation over the Equator. The air is also compressed as it descends, making the air denser and warmer as it approaches the surface of the Earth. This compression creates a high‐pressure zone of hot, dry air. On the Earth's surface, this zone is characterized by extensive deserts, such as the Sahara Desert in Africa and the Arabian Desert in Saudi Arabia.

Important characteristics of the ITCZ

Cloudiness, frequent rains, and even hostile weather. 1)One is that the trade winds flow over warm oceans across much of their length; therefore, a great deal of evaporation from these surfaces takes place, and the air contains abundant moisture. In this context, high levels of atmospheric moisture can be reached because the air is warm and expanding. 2)Second, due to the warmth of the air, the ITCZ is a place where the air lifts from the Earth's surface. As the air rises, it cools and promotes clouds and rainfall because water condenses at lower temperatures.

A dynamic convection loop

Cyclones and anticyclones are linked together in a convection loop consisting of air masses that spiral due to the Coriolis force. Note how the air masses move vertically within the high and low pressure systems and horizontally between them.

Subtropic high - Southern side

Diverges at earth's surface. *As it diverges on the southern side of the system (in the Northern Hemisphere), it flows back to the Intertropical Convergence Zone, forming the northeasterly trade winds. That is, the air flows from a zone of high pressure, the subtropical high, to a zone of low pressure, the Intertropical Convergence Zone. This movement and difference in pressure between the two zones causes wind. In the Southern Hemisphere, air flow diverges on the northern side of the system and forms the southeasterly trade winds.

Atmospheric pressure systems

In addition to the pressure changes that occur with respect to altitude, air pressure also varies horizontally across earth's surface, with distinct zones of high and low air pressure.

Cyclonic-anticyclonic flow

In general, high and low pressure systems occur next to one another and form large-scale circulatory networks that are interconnected. (Remember that air pressure is low in the center of a cyclone and high in the middle of an anticyclone.) -Because air rises in the center of a low, a void is created that must be filled. -This void ultimately becomes filled by air that flows outward from the center of the high and into the low. We feel this exchange of air as wind.

Conditions during non-ENSO years

In normal, non-El Niño conditions [above], the trade winds blow toward the west across the tropical Pacific Ocean. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees Celsius higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper layers. This cold water is nutrient-rich, supporting a diverse marine ecosystem and major fisheries. Rainfall is found in rising air over the warmest water in the western Pacific, and the east Pacific is relatively dry.

Isobars

Lines of equal pressure *A steep pressure gradient exists in regions where isobars are closely spaced together, whereas the gradient is shallow in places where the isobars are far apart. *isobars are widely spaced. This means that very limited pressure change occurs across this portion of the Earth's surface at this particular point in time. Thus, the pressure gradient in this part of the circulatory system is shallow. If you were in this area on this particular day, you would find the winds to be light, because air flowing into that region would be moving slowly *isobars become closer together. Given that isobars represent locations of equal atmospheric pressure, this close spacing can only mean that a rapid change in surface pressure occurs over a relatively small geographical space. In this area you would notice strong winds.

Ocean major type of movement: Deep currents

Movement of streams of water through the ocean. Vertical and horizontal currents are set in motion by surface contrasts in temperature and salinity (concentration of dissolved solids, expressed as parts per thousand parts of water) and by the force of the wind.

Wind directions

Northerlies: originate in the north and brings air towards the south Easterlies: originate in the east and brings air towards the west Westerlies: originate in the west and brings air towards the east Southerlies: originate in the south and brings air towards the north

Local wind systems

On a smaller scale, there are local wind systems that result from topography, proximity to large bodies of water, or other geographical features, such as: land-sea breezes and cold-air drainage (katabatibc winds)

Subtropic high - north side

On the north side of the subtropical high, air diverges and flows to the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere, forming the westerly winds or westerlies (remember, winds are named for the direction from which they flow). The westerlies dominate in the midlatitudes, specifically the movement of the fronts that bring our day-to-day weather.

Ocean major type of movement: Surface wind currents

Oscillary motions of the surface water. The surface currents of the oceans are related to the wind and pressure systems of the atmosphere. In the Northern and Southern Hemispheres the currents move in circulatory patterns around the subtropical highs (high pressure cell caused by Earth's rotation).

How Coriolis force influences wind

Remember that at ground level the direction of airflow is influenced by the pressure gradient force and results in airflow that is perpendicular to the isobars.Once the air rises into the upper troposphere, however, its speed increases because it flows freely without obstruction. Once this altitude is reached, the air is influenced most directly by the Coriolis force, which causes winds to spiral to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The force of friction

Results in airflow that is somewhere between the flow driven by the pressure gradient (perpendicular to isobars) and the coriolis force (parallel to isobars). Strongest at the surface and diminishes progressively to an altitude of about 1500m. At higher altitudes, however, wind follows a geostrophic course that is parallel to isobars.

Land-sea breezes

Results of the differential heating and cooling of land and water. *During the day: the land and the air above it is heated more rapidly than air over the sea. Thus, the air pressure over the sea is higher because the air is cooler and therefore denser. This results in a sea breeze: the air moves from sea toward land, or from high to low pressure. *At night: the air over the land cools more rapidly than the air over the sea, causing higher pressures to exist over land. Now the air reverses direction and moves from land toward sea in a land breeze.

Intertropical convergence zone

The air in this zone was carried in by the tradewinds, which traveled across warm ocean bodies. As a result, the warm air is also holding a great deal of moisture. The air cools as it rises, and remember cooler air cannot hold as much moisture. This in turn produces rainfall. It is characterized by calm winds, cloud cover, and high rainfall. (30 degrees North, and 30 degrees South).

Air pressure

The atmosphere is made up of a variety of gases that we collectively call "air." Air is half to earth by gravity and thus has weight. The weight of the air exerts pressure on the earth's surface. Varies with altitude and temperature.

Convection and advection

The combined processes of convection and advection represent the first stage of atmospheric circulation on Earth and are the overall method through which heat is distributed around the planet. Once the air is set in motion in this manner, the other forces—pressure gradient, Coriolis, frictional—then directly influence the movement of the air. The convection loops consist of spiraling masses of descending or rising air that is linked horizontally by advection. This process is most pronounced at higher latitudes and generally results in westerly airflow from the subtropics to the poles.

El Nino in the Eastern Pacific

The deep upwelling ocean current that normally dominates off the South American coastline is overtaken by the warm surface waters, which pushes the thermocline deeper than usual in the east as surface water temperatures here can rise up to 8° Celsius (46° Fahrenheit).

Example of unequal heating

The difference that exists between the tropics and the poles. The tropics are much warmer than the poles because the equatorial regions receive the most direct insolation throughout the year. If no mechanism existed to balance this difference, then the tropical and polar regions would become excessively hot and cold, respectively.

Thermohaline circulation

The global oceanic circulatory system driven by differences in water density. Regional differences in water density occur as a result of varying temperatures and salinity. Basically, water at the surface is warmer and less salty than deeper ocean waters.

Atmospheric pressure

The primary impact of airflow is to move heat energy around the globe in a way that moderates temperature on earth. Differences in pressure create both global and local winds. Directly influences the character of large and small scale wind patterns. These effects are reflected in weather and climate. Closely associated with temperature and density of air.

Ocean major type of movement: Tidal

The rhythmic rise and fall of tides. Just about everywhere the surface of the sea rises and falls twice every day. Tides are caused by the gravitational attraction of the moon and to a lesser degree, the sun.

Monsoon winds

The seasonal change in wind direction that occurs in subtropical locations due to the migration of the Intertropical Convergence Zone (ITCZ) and Subtropical High (STH) Pressure System (the unequal heating of land and water).A cyclical shift of the prevailing wind direction that occurs at a subcontinental scale over the course of a year. This process is a good example of how closely geographic variables are integrated because it is related both to the seasonal migration of the ITCZ and the maritime vs. continental effect.

El Nino in the Western Pacific

The thermocline becomes shallower in the west as the surface water cools. Cool, nutrient-rich water is no longer able to upwell along the coast in the east during an El Niño year. This has a significant impact on the coastal environment of Peru and Ecuador (as well as the climate of eastern Australia and beyond).

Unequal heating of land surfaces

The ultimate cause for all wind patterns on Earth is the unequal heating of surfaces that results from variations in the amount of solar radiation received between latitudes. This spatial variation in surface air temperature means that air density (and thus pressure) differs from place to place. At a fundamental level, surface air flows from areas of high pressure to low pressure because the atmosphere works to balance the difference between the two areas. The process of unequal heating causes motion of the atmosphere through the process of convection.

Convection

The vertical movement of air (also liquid) due to differences in temperature. Occurs in the atmosphere when air is heated, as it is in the earth's equatorial locales, causing air molecules to expand and rise. As it travels north or south towards the poles, the air cools and falls.

Trade winds

The winds that converge at the equator. As they pass over the warm oceans on their way to converging at the equator, they pick up a great deal of moisture. These winds flow to the southwest in the Northern hemisphere and to the northwest in the Southern hemisphere. This leads to increased precipitation in the region.

Cool-air drainage (katabatic winds)

"Descending," this process is most common during the winter months, when extremely cold air accumulates over higher‐altitude regions covered by ice sheets. This extremely cold, dense air then flows downhill under the force of gravity. These winds sometimes flow at great speeds that can be quite destructive, especially where the air is funneled down a narrow valley. Although the air associated with katabatic winds typically warms when descending, it usually remains colder than the air that it replaced.

Polar front

Around 60° north and south latitude, warm moist air from the westerlies meets cold, dry air from the poles. The boundary between these two contrasting air masses is known as the polar front. The cold, dry air forces the warm moist air upward so that cooling and precipitation to occur. This is one reason that the northwestern regions of the United States and Europe are cool and wet (more on this in an upcoming lesson). In the Southern Hemisphere, the polar front causes strong winds in Antarctica.

Downwelling current

As water is circulated into the higher latitudes, a downwelling current occurs due to decreasing temperature and high salinity. The dense water sinks downward, deep into the ocean, as it does along the coast of Antarctica.

Upwelling current

As water moves into the tropics, it warms and produces an upwelling current whereby less dense water rises from the depths of the ocean. The upwelling water is cooler than the surrounding surface water and nutrient-rich. These currents are found on the Pacific coast of the Americas.

Air density, altitude, and atmospheric pressure

At low altitudes air molecules are held close to earth by gravity and thus are denser, resulting in high atmospheric pressure. In contrast, the density of air molecules is low at high altitudes, and air pressure is thus relatively low.

Deep currents

Deeper currents include a vertical movement of water driven by thermohaline circulation.

Doldrums

Winds are fairly calm due to the even pressure gradient and the rising air.

Atmospheric pressure and temperature

*Air is warmed at the surface, molecules spread out, which makes the air lighter (less dense) than the surrounding air. In this case, low pressure means that there are fewer air molecules in a given unit of space. *Cold air, on the other hand, is denser (molecules are held close together). This difference means that cold air tends to sink, while warm air tends to rise.

Winter (dry) monsoon

*During the winter, a strong high pressure anticyclone cools the northern part of the Asian landmass. *At the same time, the Intertropical Convergence Zone is located over the Indian Ocean creating an area of low pressure. *As a result of this steep pressure gradient, cold, dry surface winds blow from the interior of the Asian landmass toward the coast.

Geostrophic wind formation

1)Initially, air will start to move in the direction of the pressure gradient, that is, from high to low pressure. 2)At some point, the Coriolis force will take over and force wind to the right in the Northern Hemisphere. This is more pronounced at higher latitudes. 3)Ultimately, the pressure gradient force and the Coriolis force balance each other out, and wind will blow parallel to the isobars.

Polar high

Air that flows northward at the polar front cools considerably and subsequently descends at very high latitudes, producing a weak high pressure system. This system is called the Polar High. The Polar High is a mass of descending air that rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, directing cold, dry air toward the polar front. The resulting winds are called polar easterlies. (The Polar High located over northern continental regions of the Earth makes northernmost Canada and Siberia bitterly cold and dry during the winter.)

Coriolis force and PGF

Combine to create winds called geostrophic winds.

Coriolis force

Created by earth's rotation which causes air to be deflected as earth moves below. The force is deflected to the right (west) in the Northern Hemisphere and to the left (east) in the Southern hemisphere.

La Nina

During La Niña years, a pool of very cool water shifts from the eastern tropical Pacific to the west. El Niño and La Niña represent nearly opposite sides of the ENSO climatic cycle. Consequently, sometimes El Niño is referred to as the warm phase of the ENSO cycle and La Niña the cool phase.

Position of the ITCZ in January and July

Follows the zone of most intense radiation, which varies according to the time of year due to the earth/sun geometry. January: migrates south following the shift of the sun's most direct rays. July: migrates north following the shift of the sun's most direct rays.

Key points: El Nino event

Precipitation is abundant over the central Pacific and drought conditions develop in the western tropical Pacific.

El Nino/La Nina and people

Progress has been made linking El Niño/La Niña events to natural hazards/disasters, such as unseasonable weather, floods, droughts, hurricanes, tornadoes, wildfires, geomorphic mass wasting, coastal and river erosion, devastation of the Pacific fishing industry, disease outbreaks, and so on... all of which are examples of ways that the ENSO cycle can affect people. Play a major role in global environment and influence many aspects of our lives.

Geostrophic winds

Westerly airflow from the subtropics to the poles. At this point the coriolis force and the pressure gradient force effectively balance each other, resulting in upper Air flow that moves parallel to the isobars rather than perpendicular as seen at the surface. The net result of this flow pattern is that air moves around pressure systems in the upper atmosphere.

Southern Oscillation

the atmospheric pressure conditions corresponding to the periodic warming of El Nino and cooling of La Nina. These shifts in oceanic circulation are linked to shifts in atmospheric circulation in the tropical Pacific, but their effects (altered rainfall and wind) can be felt around the world.

El Nino years

trade winds weaken and sometimes reverse. This weakening or reversal of the trade winds prevents warm surface water from mounding up in the western Pacific. Instead, this warm surface water migrates east towards the South American coast (around Ecuador and Peru). *The warm water's eastward journey takes a couple of months and results in a lowered ocean surface in the west and an elevated and warm ocean surface in the east. *High barometric pressure develops over northern Australia and a deepened low-pressure trough forms over the eastern tropical Pacific.